1. Field of the Invention
The present invention relates to a lens-frame moving mechanism which forwardly or rearwardly moves a lens group in the ready-to-photograph position by a motor-driven leading screw, and which moves the lens group between the ready-to-photograph position and the retracted position.
2. Description of the Related Art
In a known type of a mechanism for moving a focusing lens group in the optical axis direction, a focusing lens group frame is movably biased toward an object to be photographed (hereinafter, the object side), and a nut member is screw-engaged with a leading screw, which is rotatably driven by a motor, so that the nut member does not rotate relative to the focusing lens group frame and presses against the focusing lens group to move the focusing lens group against the biasing force. This known structure is disclosed in Japanese Unexamined Patent Publication No. 2009-134249.
According to Japanese Unexamined Patent Publication No. 2009-134249, the rotation of the motor is transmitted via a gear train to a leading screw, enabling the length (thickness) of a lens-frame moving mechanism along an optical axis to be reduced.
However, in a lens-frame moving mechanism using a gear train, noise caused by the rotation of the gear train is inevitable. In particular, in a camera provided with a motion-picture function, this noise is undesirably recorded during the recording of a movie. In order to reduce such noise, it is preferable that a motor driving shaft be a leading screw, and a nut member be directly screw-engaged with the leading screw, without using the gear train.
On the other hand, the above-described mechanism (motor shaft leading-screw mechanism) in which a focusing lens group (frame) is moved by the nut member that is screw-engaged with a motor shaft leading screw (i.e., the leading screw is formed on the shaft of the motor), faces the following problems. Namely, in a miniaturized camera, it is general design practice to position a focusing lens group at a position closest to the image-side of the photographing lens system, and it is desirable for the focusing lens group to be moved along the optical axis toward the image side to attain the retracted position of the photographing lens system. Secondly, in the photographing lens system having a longer backfocus, the traveling distance of the focusing lens group from the retracted position to the ready-to-photograph position (the short focal-length extremity or the long focal-length extremity) inevitably becomes longer (i.e., the traveling distance of the focusing lens group toward the image side from the ready-to-photograph position to the retracted position becomes longer).
In the above-explained conventional lens-frame moving mechanism, the longer the traveling distance of the focusing lens group from the ready-to-photograph state to the retracted state, the longer the length of the leading screw. Consequently, miniaturization of the lens-frame moving mechanism in the optical axis direction becomes difficult.